The present invention relates to a method of making a transmission mode semiconductor photocathode and particularly to a method of making such a photocathode without impairing the radiation transmission properties of the photocathode.
Transmission mode photocathodes are devices which emit electrons from one surface in response to incident radiation which passes through the device from a surface opposite the emissive surface. Certain single crystalline semiconductor material, such as gallium arsenide, indium gallium arsenide, gallium phosphide and indium arsenide phosphide, are known to be suitable for the active region of such photocathodes, particularly if its emissive surface has a negative electron affinity. To achieve efficient emission of electrons generated in the active region it is generally necessary to make the active region relatively thin, generally about one micron in thickness. Since such a thin region of the semiconductor material is not self supporting it is the practice to form the active region on a supporting substrate body, such as by epitaxially depositing the active region on the substrate body. The substrate body must be of a material which is transparent to the radiation and which will nucleate the epitaxial growth of the single crystalline material of the active region. Certain single crystalline semiconductor materials, particularly certain of the group III-V compounds and alloys of such compounds, have been found to be suitable for use as the substrate body. If the material of the substrate body has a crystal lattice which is substantially different from the crystal lattice of the material of the active region a transition region of a single crystalline semiconductor material may be provided between the substrate body and the active region to provide an active region of good crystalline quality. The transition region may be of a graded composition, such as described in an article by D. G. Fisher et al., "Negative Electron Affinity Materials For Imaging Devices" in Advances in Images Pickup and Display, Vol. I, Published by Academic Press, Inc. 1974, on page 111, or may include growth interfaces as described in U.S. Pat. No. 3,862,859, to M. Ettenberg et al., issued Jan. 28, 1975, entitled "Method of Making A Semiconductor Device ", to achieve its desired function.
In making such a transmission mode semiconductor photocathode, the transition region, if used, and the active region are epitaxially deposited on the substrate body by either of the well known processes of vapor phase epitaxy or liquid phase epitaxy. In both of these processes the substrate body is subjected to a relatively high temperature, e.g. 900.degree.C or above. Many of the semiconductor materials used for the substrate body, particularly the group III-V compounds, include a volatile element which may vaporize at the temperatures used in the deposition process causing disassociation of the material at the surface of the body. Such disassociation of the material of the body at the radiation incident surface of the body would impair the optical properties of the body. For example, if a substrate body of gallium phosphide is used, the phosphorus, which has a relatively high vapor pressure, would vaporize leaving surface faults and opaque gallium on the surface, both of which would impair the radiation transmissive properties of the substrate body. If the amount of radiation which can enter the active region through the substrate body is reduced by the impaired optical properties of the body, then the efficiency of the photocathode is reduced and the imaging quality of the device will be impaired. Therefore, it would be desirable to have a method of making the semiconductor photocathode which would not impair the optical properties of the device.